Old papers are first of all converted into the state of pulp by an apparatus commonly referred to as pulper and this pulp is then depleted from its various foreign bodies, referred to as contaminants, by causing it to pass through sieves. However, after this elimination of the contaminants, the pulp contains ink particles which should be eliminated in order to obtain a quality paper.
The pulp depleted from the coarse foreign bodies comprises three types of components, namely the fibers, the fines which are the complement to the fibers of the non-mineral portion, and the fillers which constitute the remaining mineral portion.
One step of the paper pulp treatment is the de-inking which generally comprises the injection of air into the pulp, in order to create air bubbles on the walls of which the ink particles are fixed. Through this operation of ventilation, a foam formed of bubbles the walls of which retain ink particles is caused to appear at the surface of the paper pulp. This foam is then evacuated.
Generally, in order to increase the probability of collecting the ink particles, the time of contact between the pulp and the bubbles is increased and, to this end, it is desirable to have a flow of bubbles and pulp which mix as soon as possible, and bubbles which remain as long as possible in the pulp flow. A small size of the bubbles also allows to increase this probability of collecting.
To optimize the treatment according to this knowledge, from the applicant's French patent application FR 9402886 is known a process for de-inking by means of air bubbles consisting in creating in an enclosure, by successive re-flowing of the paper pulp from one stage to the next one, from the top to the bottom of the enclosure, a downward movement of the paper pulp, this movement being crossed by an upward movement of bubbles, so that at each stage the current of pulp is crossed by a current of bubbles almost perpendicular to the current of pulp.
This process is implemented in a flotation cell the main role of which is to eliminate the ink and to extract the foam formed at the surface. However, even though the air bubbles collect the ink particles which are hydrophobic, they also draw with them some fibers which are then eliminated together with the foams.
In order to limit the fiber losses during this treatment by flotation, the applicant provides, in his French patent application FR 9 715 839, a process with counter-washing of the bubbles by means of a water supply in the upper portion of the enclosure of the flotation cell.
Also, in European patent EPO 674 040 are provided, at each stage, inlets/outlets to a paper pulp recycling system, each recycling system being provided with a pump for re-injecting the paper pulp into the lower stage. Pumping causes the speed of the paper pulp to accelerate, in order to reach the speed necessary for the operation of the device.
Even though these flotation cells are generally satisfactory, their energy consumption is relatively high. Furthermore, most of the waste is produced during the flotation and it would be desirable to reduce this waste which is generally comprised of 10% of fibers, 30% of fines and 60% of fillers. Especially the loss of fibers should be limited, in order to save noble raw material.